This invention relates to an apparatus for adjusting the position of a deflector on a television tube, particularly a color tube of the shadow mask type.
It is known that the reproduction of the images is obtained in television by displacement of a luminous dot along horizontal lines from left to right and from top to bottom, the intensity and color of the dot being a function at each instant of the intensity and color of the corresponding point of the image which is to be reproduced. To this end, a television tube comprises a screen on which are deposited luminophoric substances which emit light when they are struck by an electron beam. This beam is generated by an electron gun within the tube and the displacement of the electron beam; along the lines, for displacement of the luminous dot, is obtained by varying the magnetic fields produced by variable currents in coils known as deflectors or deflection yokes.
The positioning of the deflector with respect to the tube should be made with particular care, since any error in this positioning is manifested by an alteration of the image reproduced on the screen.
The adjustment of the deflector position with respect to the tube is particularly critical, especially for color television tubes, particularly if an automatically convergent system is required, which does not necessitate corrections performed by the circuits associated with the deflector-tube assembly. In a color television tube, the screen is actually coated with triplets of luminescent substances, each element of the triplet having one of the primary colors red, green and blue, and three electron guns are provided, each gun being intended to energize a particular color. Thus, in an autoconvergent tube, it is the deflector which assures convergence of the three electron beams throughout the screen surface. Furthermore, each electron beam should reach only the phosphors of the corresponding color. The convergence is effected by a displacement of the deflector parallel to the screen, while the purity of the color (the beam intended for one color only reaching the phosphors of this color) is adjusted by displacement of the deflector perpendicular to the screen.
Until now, these adjustments are peformed either in wholly manual manner, for example by means of rubber wedges between the deflector and the tube, or by means of a mechanism. In this latter case, the deflector is mounted on a platform displaceable with respect to a frame to which is secured the tube as such, and an operator performs the adjustment by displacement of the platform within several degrees of freedom, until an image having a minimum of defects is obtained on the screen.
The means permitting the control and displacement of the deflector support are formed by a mechanism comprising a carriage and a rocker, which is comparatively complicated. Furthermore, the adjusting operation is difficult since it causes several parameters to come into play at the same time; moreover, it is tiresome since the operator must watch the luminous screen constantly.
To eliminate these shortcomings, an apparatus has already been proposed (U.S. Pat. No. 4,360,839) for adjusting the position of a deflector with respect to a color television tube of the autoconvergent type, which comprises a support movable along n degrees of freedom and n arms of which the individual displacement is operated along a single degree of freedom, for example by simple translation, in order to establish the mobility of the support along the n degrees of freedom. Each arm is situated along an axis of the tube or along a perpendicular axis. Complex arrangements are provided in this known apparatus in order that the displacement of one arm affects the position of the deflector along one axis only of the tube.
Moreover the known apparatus is not well suited for mass production because the mounting and the removal of the tube-yoke assembly are not operations easy to perform; the mounting and the removal of the energizing means of the tube - the necessity of which results from the necessity to check on the screen that the adjustment has been correctly done are also uneasy operations.
The apparatus in accordance with the invention does not have these disadvantages.